Conventionally, for example, projection type image display apparatuses such as a projector, a rear projection television set and the like are widespread. A projection type image display apparatus is provided with a projection lens that projects an image which is formed by a reflection type image forming device such as a digital micro-mirror device (DMD) or the like or by a transmission type image forming device such as a transmission type liquid crystal device or the like. The projection lens that the projection type image display apparatus includes is usually disposed in the apparatus with held by a lens hold frame.
In such a projection type image display apparatus, during projection, the temperature of the projection lens and the temperature of the lens hold frame that holds the lens are raised by the energy of light from a light source in some cases. As an influence of this, for example, the refractive index of the projection lens changes, or the lens hold frame thermally expands, so that the position of the projection lens deviates in some cases. In such a case, a focus deviation of an image projected onto a screen occurs, which is a problem. Especially, in a projector and the like in which high resolution is required, anomalous dispersion glass whose refractive index is considerably changed by heat is used for the projection lens in many cases, the above focus deviation tends to become considerable.
Regarding this point, measures against the focus deviation due to a temperature change of the lens are conventionally reported. For example, in a patent document 1 proposes a structure in which a means that detects a temperature and generates a signal corresponding to the temperature is disposed, so that a focus change is not caused by the temperature. This structure is a structure shown in FIG. 10, for example.
In FIG. 10, 101 indicates a projection lens group; 102 indicates a temperature detection mechanism that detects a temperature in the vicinity of the projection lens group 101 and outputs a signal corresponding to the detected temperature; 103 indicates a focus correction amount calculation means that decides a focus correction amount based on the temperature detected by the temperature detection mechanism; 104 indicates a focus correction member drive mechanism that performs a focus adjustment of the projection lens group 101 based on the correction amount decided by the focus correction amount calculation means 103.    Patent document 1: JP-A-1984-137917